A 3-dimensional measuring apparatus or a circularity measuring apparatus is known in the art for measuring a coaxial relation between two mechanical parts which are arranged to have a common axial line.
In these prior art measuring apparatuses, a contact type measuring device is moved on measuring objects, being in contact with the measuring objects, to define configurations of those mechanical parts and finally measure the coaxial relation between those two mechanical parts.
It is, however, disadvantageous in the above prior art apparatuses using the contact type measuring device, in that it would become difficult or impossible to define the configuration of the parts and to measure the coaxial relation in the case that the parts have complicated configuration. For example, in the case that the part of the measuring object has a long cylindrical portion and a cylindrical inner surface with a small diameter will be measured, a measuring device having a slender arm will be necessary. However, when the measuring device is made longer, a rigidity of the measuring device is decreased to decrease measuring accuracy.